fix: migrate UploadPage to V2 API and fix logout navigation

Changes:
- Add uploadFile() method to apiClientV2 for single file uploads
- Update UploadPage to use apiClientV2 instead of apiClient
- Change upload logic to iterate files and collect task IDs
- Add navigation to /login after logout in Layout component

Fixes:
- 403 Forbidden error on file upload (token mismatch between V1/V2 APIs)
- Logout button not redirecting to login page after clearing auth

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/components/Layout.tsx

@@ -1,4 +1,4 @@
-import { Outlet, NavLink } from 'react-router-dom'
+import { Outlet, NavLink, useNavigate } from 'react-router-dom'
 import { useTranslation } from 'react-i18next'
 import { useAuthStore } from '@/store/authStore'
 import { apiClient } from '@/services/api'
@@ -20,6 +20,7 @@ import {
 
 export default function Layout() {
   const { t } = useTranslation()
+  const navigate = useNavigate()
   const logout = useAuthStore((state) => state.logout)
   const user = useAuthStore((state) => state.user)
 
@@ -38,6 +39,7 @@ export default function Layout() {
       console.error('Logout error:', error)
     } finally {
       logout()
+      navigate('/login')
     }
   }